1. Field of the Invention
The present invention relates to a thermal head of a partial-glazed type, and more particularly to a protruding structure of a heater portion on the partial glaze.
2. Description of the Related Art
Conventionally, in order to avoid the accumulation of heat on a head, a resistor (a heater) was provided on a substrate formed by a material which has a large thermal conductivity. As a result, nearly 80% of input energy was radiated, and only 15% or less of heat was able to be contributed to recording. Conversely, in a so-called thermal head of a partial-glazed type in which a resistor was fixed by a fused glass (glaze) having a small thermal conductivity, as compared to a thermal head without a glaze layer, the temperature of the resistor was able to be raised and a large amount of heat was able to be contributed to recording.
As shown in FIG. 6, a partial glaze 1 is provided at the recording material opposing surface S (a lower surface in FIG. 6) side of a thermal head 5 which includes a radiation fin 3. A heater portion is formed at the top surface of the partial glaze 1. In general, radius of curvature of the heater portion is 20 mm, and the height of the partial glaze 1 from the recording material opposing surface S of the thermal head 5 is about 60 .mu.m. The enlarged view of the partial glaze 1 in FIG. 6 is shown by 1A in FIG. 8A. Additionally, FIG. 6 shows an IC cover 7 and a peel bar 9.
In a thermal printer having such thermal head 5, an unillustrated platen is provided so as to oppose the partial glaze 1, and an unillustrated recording sheet and a toner ribbon are superposed so as to be inserted between the partial glaze 1 and the platen. The portion of the partial glaze 1 corresponding to an image to be printed is selected and heated, and the image is transferred to the recording sheet due to the heat transfer from the toner ribbon.
However, in the aforementioned conventional thermal head 5, heat is transmitted to the recording material as the partial glaze 1 contacts the recording material. Accordingly, when a foreign object is disposed at the recording material, the foreign object moved by the conveyance of the recording material catches on the partial glaze 1, and the portion of the partial glaze 1 on which the foreign object catches does not contact the recording material. Thus, there is a drawback in that heat transfer is not effected on the portion of the recording material at which the foreign object passes and that a void portion (a so-called line void) occurs.
On the other hand, in order to solve the drawback, as shown in FIG. 7, a thermal head, in which the radius of curvature of the heater portion on the partial glaze 1 is 50 mm and the height of the partial glaze 1 is 20 .mu.m, may be used for reducing the catching of a foreign object. The enlarged view of the partial glaze 1 in FIG. 7 is shown by 1B in FIG. 8A. However, in this case, the radius of curvature of the heater portion is 50 mm and the heater portion is flat. Because the head pressure of the heater portion is reduced, there are drawbacks in that "irregularities" and "roughness" occur and that the image quality is deteriorated.
The drawbacks will be explained in more detail by using FIGS. 8A and 8B. The width of the partial glaze cannot be narrowed so much due to the convenience of a manufacturing process, i.e., if the width of the partial glaze is forcibly narrowed, the height thereof cannot be formed uniformly. Accordingly, as shown in FIG. 8A, the width of the partial glaze is limited to, e.g., 1.5 mm. Therefore, the height of the partial glaze and the radius of curvature of the heater portion formed at the top surface of the partial glaze cannot be changed independently. As shown by C in FIG. 8B, ideally, the height of the partial glaze is 20 .mu.m while the radius of curvature of the heater portion is kept 20 mm. However, this cannot be effected due to the convenience of the manufacturing process. Therefore, in reality, as shown in FIG. 8A, when the height of the partial glaze is decreased, the radius of curvature of the heater portion is increased.